A total knee replacement is a complex procedure that requires an orthopedic surgeon to make precise measurements and skillfully remove the diseased portions of a patient's bone, in order to shape the remaining bone to accommodate a knee implant. During the surgery, the surgeon makes an incision over the front of the patient's knee to gain access to the joint capsule. A capsulotomy, or incision, is made in the joint capsule to access the knee joint. Once the knee is open, the surgeon moves the patella to the side of the knee to allow the surgeon to visualize the areas needed to perform the surgical procedure. The first bone to be prepared for resurfacing is the patient's femur or thighbone. Once the surgeon has exposed the patient's knee joint, the surgeon carefully measures the patient's bones and makes precise cuts using special instruments. The damaged bone and cartilage from the end of the femur is cut away. The end of the patient's femur is shaped by performing these cuts to fit the first part of the artificial knee, the femoral component. The surgeon then addresses the tibia or shinbone. The surgeon removes damaged bone and cartilage from the top of the tibia and then shapes the bone to fit the metal component that covers the top of the tibia. The bottom portion of the knee implant, called the tibial tray, is then fitted to the tibia and secured into place using bone cement. Once the tray is in place, the surgeon snaps in a polyethylene (medical-grade plastic) insert into the tibial tray to sit between the tibial tray and the femoral component. This tibial insert functions as an efficient artificial bearing surface for the knee. Once the tibia component is cemented in place the femoral part of the implant is cemented to the end of the thigh bone. Before returning the patella to its normal position, the surgeon might need to flatten the patella and fit it with an additional plastic component in order to ensure a proper fit with the rest of the implant. The plastic piece, if needed, is cemented to the underlying bone. The surgeon bends and flexes the knee to ensure that the implant is working correctly, and that alignment, sizing, and positioning is suitable. To complete the procedure, the surgeon will close the joint capsule with stitches and the skin incision with stitches or staples. Bandages are applied and the patient is transferred to the recovery unit.
Total Knee Systems use a conventional instrument or device (FIG. 1) to assess the anteroposterior (AP) size of the femur, determine rotation of the femur, and aid in the positioning of the cutting guide. This device is large, which makes it difficult to use and requires a large incision for exposure and visualization. The current AP sizer includes multiple moving parts which are prone to move and incorrectly size the femur and malposistion the implant. The knee may need to be manipulated, flexed, lifted or moved to allow placement of the device. In smaller incisions, the device can be difficult to use and could potentially lead to misjudging the implant size, placing the patient at risk. The large device, with multiple moving parts can add additional time to the surgical procedure.